Circuit interrupter



March 17, 1953 H. L. RAWLINS ET AL CIRCUIT INTERRUPTER 2 SHEETSr-SHEET 1 Filed Jan. 4, 1947 March '17, 1953 H. RAWLINS ET AL CIRCUIT INTERRUPTER Filed Jan. 4, 1947 2 SHEETSSHEET 2 m F191;! F797. 2.

H Curt :0! y Ma de INVENTORS and 66 4/5/55 k.

Patented Mar. 17, 1953 UNITED STATES PATENT OFFICE CIRCUIT INTERRUPTEE Application January 4, 1947, Serial No. 720,166

15 Claims. (01.200-147) This invention relates to circuit interrupters in general, and more particularly to arc extinguishing structures therefor.

A general object of our invention is to provide an improved circuit interrupter particularly of. the air-break type, in which the structural positioning of the several parts and the arrangement thereof is such as to render greatly improved operating performance.

A more particular object of our invention is to provide improved means for causing a phase lag between the magnetic field and the are current to obtain an increase in interrupting ability at current zero.

Another object is to provide an improved transfer device to facilitate the transfer of one end of the are drawn by the contact structure to the arcing horn, to thereby bring about the insertion of the blowout coil into series circuit.

Another object is to provide improved plate construction for the circuit interrupter of the. type which establishes an arc and. moves thesame toward the closed ends of slots formed in. a plurality of spaced insulating plates.

Another object is to provide an improved magnetic structure to result in uniform magnetic blowout field throughout the entire length ofv the arc chute structure.

Another object is to provide improved means for disseminating the arc gases exhausting between the plate portions constituting the arc chute, to thereby prevent the possibility of flashover across the exhaust end of the arc chute.

Further objects and advantages will readily become apparent upon reading the following specification taken in conjunction with the drawings, in which:

Figure 1 is a side elevational view, partly in vertical section of an air-break type of circuit interrupter embodying our invention and shown in the closed circuit position;

Fig. 2 is a fragmentary vertical sectional view taken along the line 1III of Fig. 1, looking inthe direction of the arrows;

Fig. 3 is an enlarged fragmentary sectional view of the coil structure taken along the line.

IIIIII of Fig. 1, looking in the direction of the arrows;

Fig. 4 is an enlarged sectional view taken through the transfer interrupter device along the line IVIV of Fig. 1,, looking in the direction of the arrows;

Fig. 5 is an enlarged detail plan view of the transfer are horn taken on the line VV of Fig. 1.;

Figs. 6 through 8 are diagrammatic views illustrating the principles involved in our invention by causing a phase lag to exist between the mag.- netic field and the arc current, and

Fig. 9 is a detail side elevational view of one of the two shading coils employed in our invention.

Referring to the drawings, and more particularly to Fig. 1 thereof, the reference character I generally designates contact structure operable to draw an are which moves upwardly into an interrupting chamber generally designated. by the reference numeral 2. The are is interrupted within the interrupting chamber 2 to thereby interrupt the circuit controlled by the breaker.

More specifically, the contact structure I. includes a terminal stud 3 surrounded by a flanged insulating. bushing 4, the latter extending through a support plate 5 and secured thereto. At the right-hand end of the terminal stud 3 is secured a stationary main contact 6 having a pair of outwardly extending metallic support plates 1 secured thereto. A rotatable contact arm 8 is piv-otally mounted at 9 between the support plates I, and is caused to move thereabout by the actuation of an insulating operating rod 12.

The operating rod I2 is actuated by suitable mechanism, not shown, but which is responsive either to. manual operation or to the existence of overload conditions existing in the circuit controlled by the interrupter.

The operating rod I2 is pivotally connected at l3 between a pair of support plates [4 rigidly secured to and rotatable with the contact arm 8.

Also movable with the contact arm 8 is a main contact bridge l5 formed of conducting material and electrically interconnecting the stationary main contact 6 with a second stationary main contact IT. The contact I! is secured to the right-hand end of a second terminal stud I8, which insulating bushing 19. The bushing I9 is secured to and supported by the framework or supporting plate structure 5.

Movablewith the rotatable contact arm 8 is apiston cylinder 20 within which moves a piston, not shown, secured to a piston rod 2|, pivotally secured at 22 to the support plates 1.

It willbe apparent that upon clockwise opening rotative motion of the contact arm 8 about the pivot point 9 the piston, not shown, will move upwardly within the piston cylinder 20 to force a blast of compressed air out of the upper end of the piston cylinder 20 and through a blast tube 23 adjacent the contact structure. This blast of is likewise enclosed within a flanged air facilitates upward movement of low current arcs into the interrupting chamber 2, when, because of the low currents involved, the magnetic field is relatively weak.

The piston construction is set forth and claimed in United States patent application filed August 22, 1946, Serial No. 692,331, now U. S. Patent 2,491,9e4, issued December 20, 1949 to Russell E. Frink and assigned to the assignee of the instant application.

Also supported by the second stationary main contact structure H are a pair of relatively stationary secondary and arcing contacts respectively designated by the reference characters 24, 25. The secondary contact 24 cooperates with a movable secondary contact 25 carried by the contact arm 8. The construction is such that the movable secondary contact 25 separates from the relatively stationary secondary contact 24 after disengagement of the contact bridge 15 from the stationary main contacts 5, ll. However, the secondary contacts 24, 26 separate prior to the separation of the arcing contacts 25, 27:. Thus, the are which is finally drawn by operation of the contact structure I is drawn between the arcing contacts 25, 2?.

Because of the loop circuit formed by the contact structure 1 including the horizontally disposed terminal studs 3, l8 and the moving arm 8, the are which is finally drawn between the arcing contacts 25, 2'1 expands upwardly to thereby cause one portion of the drawn arc to engage the panel arcing horn or are terminal member 35. The are position at this point is indicated by the dotted line 32 with the movable arcing contact 2'! being moved to the position shown in dotted lines in Fig. 1.

We have provided suitable coil structure 33, including a, pair of parallel disposed blowout coils 34 more particularly shown in Figs. 1 and 3. One end of the coils 34 is connected to the socket stationary contact 35. The other ends of the coils 34 are connected to the stationary main contact structure I1 by the strap connection 34a. Thus upon transfer of one terminal of the are 32 to the arcing horn 33, the coil structure 33 will be inserted into series circuit and will serve to energize the transverse magnetic field extending between the magnet pole shoes 35, 31. This transverse magnetic field extending across the interrupting chamber 2 serves to cause upward movement of the are now designated by the reference character 38 into the arc chute 39 including a plurality of spaced slotted ceramic plates 43.

It will be noted that each pole shoe 35, 31 has depending downwardly therefrom and secured thereto, such as by welding, a sub-pole piece 4! or 42.

The use of such sub-pole pieces 4!, 42 facilitates the initial upward movement of the arc initially drawn between the contacts 25, 2?. This follows inasmuch as when the arc is originally.

drawn between the arcing contacts 25, 2'! there is formed a magnetic field about said arc. A portion of the magnetic field passes through the sub-pole pieces 4|, 42 through the pole shoes 35, 31 and through the core 13 enclosed by the blowout coils 34. However, the magnetic field below the are, as initially drawn between the arcing contacts 25, 21, must pass through the air space below the arc and since the magnetic field is more concentrated below the arc than above it, the arc is rapidly moved upwardly to cause a portion of it to strike the arcing horn 3! Also the portion of the are designated by the referbythe alignment of the slots.

ence numeral 44 is moved toward the left into the transfer interrupting stack generally designated by the reference numeral 41, the construction of which is more clearly shown in Figs. 1, 4 and 5.

Referring more particularly to Figs. 4 and it will be noted that the transfer interrupting stack 41 includes a plurality of spaced slotted ceramic plates 48 having slots 49 provided therein. The closed ends of the slots 49 are slightly unsymmetrical about the center line 53 of the plates 48 so that the alternate positioning of the plates 48 will cause a zig-zag arc passage 5! to be formed The off-setting of the closed ends of the slots 48 to about the center line 58 causes an elongation of the arc portion 44, as it is moved toward the left into the transfer interrupting stack 41.

We have provided a transfer are horn 54, the configuration of which is more clearly shown in Fig. 5. The transfer are horn 54 is of metallic material, preferably copper and is secured by two screws of copper passing through countersunk apertures 55 provided in the arc horn 54 to terminate in the upper end of the stationary main contact plate 55 (Fig. 1).

We have provided a plurality of slots 51 in the arcing horn 54 to facilitate the leftward motion of the are along the arcing born 54 from the stationary arcing contact 25.

Below the arcing horn 54 and secured thereto by rivets 53 are a plurality of plates formed of magnetic material such as steel. In this instance, there are four such magnetic plates, two of which are designated by the reference character 59 and two of which are designated by the reference character 65. The presence of the four plates 59, 50 of magnetic material disposed immediately below the transfer arc horn 54 facilitates the leftward movement of the lower end of the arc from the stationary arcing contact 25 to the are horn 54 to facilitate the entering motion of the arc portion 44 of the are 32 into the transfer interrupting device 47.

High field strengths have been found necessary for high interrupting capacity within a small space. All of the flux across the interrupting chamber 2 between the pole shoes 36, 31 must thread the magnetic coils 34. This means that the reactance of the magnetic coils 34 is high and a countervoltage of several hundred volts will appear across the terminals of the magnetic coils 34 when the short circuit current is attempted to be transferred to the coils 34, which are in parallel, and which are connected electrically between the arc horn 30 and the upper terminal stud l8.

This makes the transfer interrupter 47 necessary to interrupt the arc portion 44 extending between the transfer are horn 54 and the arc horn 30.

The use of a transfer interrupting device 41 to facilitate insertion into series circuit of the blowout coils 34 is more fully set forth and claimed in U. S. patent application filed January 4, 1947, Serial No. 720,165, now U. S. Patent 2,616,007, issued October 28, 1952 to Robert C. Dickinson and Russell E. Frinlr and assigned to the assignee of the instant application. This application also sets forth and claims the use of the sub-pole pieces 4|, 42 to facilitate the initial upward movement of the are as initially drawn between the arcing contacts 25, 21.

After the transfer interrupting device 4'! has interrupted the arc portion 44 and caused the are. toextend between the are born. 39'and" the movable arcing contact 21., the paralleled dis:- posed blowout coils 34 are inseries circuit; and energize the transverse magnetic field extending across the interrupting chamber. 2 through. the pole shoes 33, 3?. The. right-hand end ofvv the are 32 soon transfers to the right-hand arc horn or are terminal member 92 which is connected by a conducting strap 63 to the lower terminal stud 3 of the interrupter. Thusv the reference character 38 indicates the position of the arc extending between the arc horns 39, 62 after the. blowout coils 34 are in series circuit.

The arc 33 is moved upwardly within the arc chute 39, which as mentioned previously, includes a plurality of spaced insulating ceramic plates or plate portions 49. Fig. 2 more clearly shows the configuration of the plates 40.

It will be noted that. each plate or plate portion 40 has a slot 64 provided therein, the upper closed end of which is slightly unsymmetrical about the center line- 65 of the plate 49. Thus the alternate positioning of the plates 48 about the center line 65 of said plates produces a zig-zag arc passage 66 at the upper closed ends of the staggered slots 64. The above staggered construction is set forth and claimed in the foregoing application by Dickinson and Frink'.

The upward movement of the are 38 into the arc chute 39 and into the upper closed ends of the slots 64 effects the restriction thereof and also an elongation. thereof. The transverse mag netic field extending between the. pole shoes 39, 31 imparts'to the electrons an upward component of velocity. These electrons bombard gas particles and produce an actual blast of gas perpendicular to the arc, which action requires the arc to ionize fresh quantities of gas in considerable quantities. When current zero is reached, this action is coninued to effectively deionize the arc and establish dielectric in the arc chute 39. The theory of the interrupting operation is more clearly set forth in U. S. patent application filed December 15, 1943, Serial No. 514,362,. new U. S. Patent 2,442,199, issued May 25, 1948, to Robert C. Dickinson and Russell E. Frink and assigned to the assignee of the instant application. This patent also claims the plate structure disposed adjacent to the arc horns 30, 62.

We have discovered that by shortening the end plates 40' as indicated by the reference characters 61 the arc gases passing between said plates and the arc horns 30, 62 diverge toward the sides of the arc chute as indicated by the arrows 69.

Thus the shortening of the end plates 40 causes the arc gases adjacent the upper ends of these plates to diverge outwardly and not to commingle with the associated arc gases pass ing upwardly between the other remaining insulating plates 49 as indicated by the vertical arrows T9. The result of such. a construction is to cause a divergence of the associated arc gases at the upper end of the arc chute 39 to minimize the possibility of fiashover across the. upper end of the arc chute 3.9.

It will be observed from the above discussion that interruption depends on the rate of deionization at current zero, andif a higher rate could be maintained at this instant the interrupting capacity will be increased. If the magnetic flux is made to lag the arc current by a small angle, the flux will have an appreciable magnitude. at current zero, and at the instant just. preceding it. We have discovered that. this lag: should be' approximately 10; and that when. this. lag' is introduced, a.considerable .increase in. the. interrupting ability is effected. Although. we. have found 10 to be the most. effective, a. considerable range in. the angle of. lag will still.

produce an improvement.

In now abandoned U. S- patent. application filed January 4, 1947, Serial No. 720,156, by Ros.- well C. Van Sickle and assigned to the assignee of'the instant application a resistance was used to shunt the blowoutcoil to provide a phase lagbetween the magnetic field and the arc current-.- We have improved the construction in this application by employing a shading coil ll (Fig. 9) associated with each blowout coil 34. The shading coil H is a brass Washer, inch wide and 0.040 inch thick and of the same contour as the magnet coil 34-. The lag between the magnetic field and the arc current may also be obtained by obtainin high eddy current losses in the magnet structure.

Preferably'we laminate the pole shoes 36', 3T in a horizontal direction as indicated in Fig. 2. The magnet. iron which we employ consists of a U-shaped structure composed of the magnet shoes 36,. 3'! and the magnet yoke 43 or core. The magnet shoes for the voltage. considered are 9 /2 inches high, 1% inches wide, and of suflicient length to extend from the. magnet yoke or core 43, to approximately flush with the front.

ceramic. plate 45}, disposed at the right-hand end of the arc chute. 39 in Fig. 1. The laminations. forming. the-pole shoes 36, 31 areof medium silicon steel 0.018 inch thick. Insulating tubes 72 are placed around the rivets 13 which hold the magnet shoes 36, 31 together, and insulating washers placed under the rivet heads to keep losses to a minimum in the shoes 36, 31. The magnet yoke 43 or core is made of a similar grade of silicon steel and the laminations are placed at right angles to the laminations in the shoes 36, 31. The core 43 has a length in the direction extending away from the arc-establishing means 25, 2! (that is, the vertical direction in Fig. 1) substantially the same as the length of the pole shoes 36, 31 in said direction. The rivets M in the core 43 are not insulated. The width of. the core. 43' or yoke is 1 inches as compared to the 1% inch dimension in the magnet shoes 36, 31. As the core or yoke 43. must carry all the flux which is carried by the shoes 36', 3'!- the dimensional relation between the two forms a bottleneck in the magnetic circuit-and prevents extreme unbalance in the magnetic; field due to high saturation of the shoes- 36, 31 in the region adjacent to the yoke 43.

In a magnetic breaker of the. type described, a uniform field strength in the interrupting region between the pole pieces 36 and 31 is required for optimum performance. This is necessary toeffect. uniform travel of the are throughout its length into the slots in the interrupter plates. If the field is not uniform, one of the following conditions, will. exist:

(-1.). The-structure will attempt to interrupt the arc when only that part which is in the region of higher field strengths has reached the ends of the slots. The balance of the are which is lower down in the slots will not be subjected to the same vigorous deionizing action, and the summation of the d'eioni'zingaction over the length of the arc will be less.

(2) If the design constants (field strength, plate thickness, offset, venting, etc.) are adjusted so that, even though the field strength is not uniform, the entire arc is at the top of the slots when interruption is attempted, the part in the regions of higher field strength will have been stationary at this point long enough to raise the temperature of the ends of the slots sufiioiently to impair the dielectric strength of the ceramic insulating material, and the maximum interrupting capacity will be lowered.

The magnetomotive force which produces the magnetic field in the interrupting space between pole pieces 353i is derived from the blowout coils 34. The magnetomotive force around any closed loop including the magnet core 43 is equal to the magnetomotive force in these coils. Considering the closed loops comprising the core 43, the pole pieces 36-3! and the interrupting space, there is a magnetomotive force drop in each part of each of the components which is equal to the fiux density at the point considered times the reluctance at that point times the differential length of path dl. It follows that the flux density in any part of the interrupting space is proportional to the magnetomotive force across that space which is equal to the MMF of the coil minus the MMF drop in the core minus the MMF drop in the sections of the pole pieces between the core and the point under consideration. Referring to Fig. 1, it is evident that the fiux density in the pole pieces is highest in the regions adjacent to the core and decreases progressively from left to right since any vertical section through the pole pieces on a plane perpendicular to the paper will intersect all the flux to the right of the section.

When enough field strength is provided to handle medium and low currents, there is enough MMF' available at high currents to saturate some of the iron. If too much saturation is permitted in the pole pieces, the increased reluctance due to the saturation will reduce the MMF across the interrupting space in the regions farther removed from the core and cause considerable unbalance.

However, if the cross-section of the core is made smaller than the cross-section of the pole pieces, the core will saturate and its increased reluctance will limit the flux and prevent undue saturation of the pole pieces, and the result is a uniform field over the entire interrupting space.

As mentioned above for the voltage considered, the cross section of the core is 1%; x 9 /2 inches and the cross-section of the pole pieces is 1% X 9 inches. It has been found that these proportions make it possible to use enough turns in the coil 34 to provide fields strong enough to accomplish positive and speedy interruption of low and medium current arcs and still keep the field satisfactorily uniform over the interrupting space at the very highest currents.

Preferably, we have the cross-sectional area of the core 43 less than the cross-sectional area of the pole shoes 36, 31; and our preferred range for the cross-sectional area of the core 43 is between 60% and 85% of the cross-sectional area of the pole shoes 35, 31. The best value for the cross-sectional area of the core 43 is 80% of the cross-sectional area of the pole shoes 36, 37.

Referring to Figs. 6 through 8, it will be noted that Fig. 6 diagrammatically indicates the magnetic circuit comprising the pole shoes 36, 31, the core 43 with the two shading coils 1| associated with the two parallel disposed blowout coils 34.

Let I1 be the current in the arc and blowout coil. Let on be the magnetic flux produced by I1. Let E2 be the voltage induced in the shading coil H by 931. Let I2 be the current in the shading coil H assuming resistance only. Let p2 equal the magnetic flux produced by I2, then referring to Fig. '7 it Will be observed that qua: is the fiux in the interrupting space equal to the vector sum of q l+q 2.

Referring to Fig. 8, y is the field strength at current zero. Thus it results that by the construction employing two shading coils H of configuration as shown in Fig. 9 there results the flux err in the interrupting space, and there is an appreciable field strength at current zero as designated at y of Fig. 8. a is the angle of lag between the phase of the field and are current.

Although we have shown and described a specific structure, it is to be clearly understood that the same was merely for the purpose of illustration, and that changes and modifications may be readily made therein by those skilled in the art without departing from the spirit and scope of the appended claims.

We claim as our invention:

1. In a circuit interrupter, a magnet structure including a core and a pair of pole shoes, means for establishing an are, means including the magnet structure to effect extinction of the are by moving the are within the transverse field extending between the pole shoes, and the crosssectional area of the core being less than the cross-sectional area of the pole shoes so that the increased reluctance of the core will limit the magnetic flux and prevent undue saturation of the pole shoes to thereby result in a more uniform field over the interruptin space.

2. In a circuit interrupter, a magnet structure including a core and at least one pole shoe, means for establishing an are, means including the magnet structure to effect extinction of the arc, and the cross-sectional area of the core being less than the cross-sectional area of the pole shoe so that the increased reluctance of the core will limit the magnetic flux and prevent undue saturation of the pole shoe to thereby result in a uniform field over the interrupting space.

3. In a circuit interrupter, a magnet structure including a core and at least one pole shoe, means for establishing an are, means including the magnet structure to eifect extinction of the arc, and the cross-sectional area of the core being within the range between 60% and 85% of the cross-sectional area of the pole shoe so that the increased reluctance of the core will limit the magnetic fiux and prevent undue saturation of the pole shoe to thereby result in a more uniform field over the interrupting space.

4. In a circuit interrupter, a magnet structure including a core and at least one pole shoe, means for establishing an arc, means including the magnet structure to efiect extinction of the arc, and the cross-sectional area of the core being substantially of the cross-sectional area of the pole shoe so that the increased reluctance of the core will limit the magnetic flux and prevent undue saturation of the pole shoe to thereby result in a more uniform field over the interrupting space.

5. In a circuit interrupter, means for establishing an arc, means including a magnet blowout structure for causing interruption of the arc, the blowout structure including only a single core, an energizing winding for the core, and

a short circuited winding forming a shading coil disposed only about and completely encircling the core to effect thereby a phase lag between the magnetic field and the arc current.

6. In combination with an air-break circuit interrupter, an arc chute including a plurality of spaced slotted plates of insulating material, means for establishing an arc and moving it within the slots of the plates to effect the extinction thereof, a magnet structure including a single core and at least one pole shoe to effect movement of the are within the. arc chute, an energizing winding for the core and a short circuited shading coil completely encircling the single core to effect a phase lag between the magnetic field and the arc current.

7. In a circuit interrupter of the air-break type, terminals of opposite polarity, means for establishing an are, an arc chute including a plurality of relatively ciosely spaced plates the major portion of which are of insulating material, the exhaust ends of all of the plates bein composed of insulating material, the plates being disposed substantially parallel to each other and positioned substantially transversely of the established arc, arc terminal members adjacent the ends of the arc chute and electrically connected at least durin arc extinction to the terminals of opposite polarity, means for moving the established are against the edges of the plates to effect the extinction thereof with exhaust arc gases passing outwardly between the plates, at least one of the end plates being appreciably shorter at the exhaust end thereof than most of the other plates to divert the exhaust arc gases in a different direction than that taken by the arc gases between most of the other plates to minimize the possibility of fiashover across the exhaust end of the arc chute, and said one end plate terminating at an insulating portion of the immediately adjacent longer plate.

8. In a circuit interrupter of the air-break type, terminals of opposite polarity, means for establishing an arc, an arc chute including a plurality of relatively closely spaced slotted plates the major portion of which are of insulating materail, the exhaust ends of all of the plates being composed of insulating material, the plates being disposed substantially parallel to each other and positioned substantially transversely of the established arc, arc terminal members adjacent the ends of the arc chute and electrically connected at least during arc extinction to the terminals of opposite polarity, means for moving the established arc within the slots of the plates to eiTect the extinction thereof with exhaust arc gases passing outwardly between the plates, at least one of the end plates being appreciably shorter at the exhaust end thereof than most of the other plates to divert the exhaust arc gases in a different direction than that taken by the arc gases between most of the other plates to minimize the possibility of flashover across the exhaust end of the arc chute, and said one end plate terminating at an insulating portion of the immediately adjacent longer plate.

9. In a circuit interrupter of the air-break type, means for establishing an arc, an arc chute including a plurality of relatively closely spaced slotted plates the major portion of which are of insulating material, the exhaust ends of all of the plates being composed of insulating material, the plates being disposed substantially parallel to each other and positioned substantially transversely of the established arc, means for moving the established are within the slots of the plates to effect the extinction thereof with exhaust arc gases passing outwardly between the plates, at least the end plates of the arc chute being appreciably shorter at the exhaust end thereof than most of the other plates to divert the exhaust arc gases in a different direction than that taken by the arc gases between most of the other plates to minimize the possibility of fiashover across the exhaust end of the arc chute, and said end plates terminating at an insulating portion of the immediately adjacent longer plates.

10. In a circuit interrupter, a magnet structure including a core and a pair of cooperating pole shoes, means for establishing an are between the pole shoes to be acted upon by the magnetic field set up therebetween, means including the magnet structure to effect extinction of the arc, the cross-sectional area of the core being less than the cross-sectional area of the pole shoes, and the length of the core in the direction extending away from said arc-establishing means being substantially the same as the length of the pole shoes in said direction.

11. In a circuit interrupter, a magnet structure including a core and at least one pole shoe, means for establishing an arc, means including the magnet structure to eifect extinction of the arc, the cross-sectional area of the core being less than the cross-sectional area of the pole shoe so that the increased reluctance of the core will limit the magnetic flux and prevent undue saturation of the pole shoe to thereby result in a uniform field over the interrupting space, and the len th of the core extending away from said arc-establishing means being substantially the same as the length of the pole shoe in said direction.

12. In a circuit interrupter, a magnet structure including a core and at least one pole shoe, means for establishing an arc, means including the magnet structure to effect extinction of the arc, the cross-sectional area of the core being within the range between 60% and 85% of the cross-sectional area of the pole shoe so that the increased reluctance of the core will limit the magnetic flux and prevent undue saturation of the pole shoe to thereby resuit in a more uniform field over the interrupting space, and the length of the core in the direction extending away from said arc-establishing means being substantially the same as the length of the pole shoe in said direction.

13. In a circuit interrupter, a magnet structure includin "a core and at least one pole shoe, means for establishing an are, means including the magnet structure to effect extinction of the arc, the cross-sectional area of the core being substantially of the cross-sectional area of the pole shoe so that the increased reluctance of the core will limit the magnetic flux and prevent undue saturation of the pole shoe to thereby result in a more uniform field over the interrupting space, and the length of the core in the direction extending away from said arc-establishing means being substantially the same as the length of the pole shoe in said direction.

14. A circuit interrupter of the air-break type including means for establishing an arc, an arc chute including a plurality of relatively closelyspaced plate portions the major portion of which are of insulating material, the exhaust ends of all of the plate portions being composed of insulating material, the plate portions being disposed substantially parallel to each other and .effect the extinction thereof with exhaust arc gases passing outwardly between the plate portions, at least one of the end plate portions being appreciably shorter at the exhaust end thereof than most of the other plate portions by an amount at least several times the spacing between the. plate portions to divert the exhaust arc gases in a different direction than that taken by the arc gases between most of the other plate portions to minimize the possibility of flashover across the exhaust end of the nating at an insulating portion of the immediately adjacent longer plate portion.

15. A circuit interrupter of the air-break type including means for establishing an arc, an arc chute including a plurality of relatively closelyspaced plate portions the major portion of which are of insulating material, the exhaust ends of all of the plate portions being composed of insulating material, the plate portions being disposed substantially parallel to each other and positioned substantially transversely of the established arc, means for moving the established are against the edges of the plate portions to arc chute, and said one end plate portion termi- 12 the other plate portions to minimize the possibility of flashover across the exhaust end of the arc chute, and said end plate portions terminating at an insulating portion of the immediately adjacent longer plate portions.

HERBERT L. RAWLINS. ROBERT C. DICKINSON. RUSSELL E FRINK.

REFERENCES CITED The following references are of record in the file of this patent:

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